how many and what type of sub are you gonna end up getting.
you could probably just go ahead and get a nice 5 channel amp now if you are only gonna run 1 sub.
that will save you having to get another amp and distribution block etc etc. in the long run

 

Headroom is what you want. Your amp will live longer if it doesn't have to work as hard to push your speakers. Mine for example:
tweeters: rated for 60 watts each, amped for 100 each
mids: rated for 150 watts each, amped for 200 each
subs: rated for 600 watts total, amped for 1200+.

Now, will that much power ever be put to my speakers that could potentially damage them? No, the gains are set properly. But my amps dont even get warm at ridiculous volume levels.

 

^ very much true. A added bonus is down the road when you want to get something better you'll already have the power to drive them.